Pictures apparently showing a group of urban thrillseekers posing at the top of the unfinished Shard, Europe's tallest building, have appeared online.

The night-time images, said to be from the summit of the 310-metre (1016ft) Shard in central London, were posted on Place Hacking, the blog of the US student Bradley L Garrett, who said they were taken after he slipped past a lone security guard with two friends.

Garrett said he and the group had climbed the tower "half a dozen" times and could do so again tomorrow in spite of increased security.

One of the images appears to show a man sitting in the cab of a crane at the top of the giant glass skyscraper, which is due to be completed in June, while others show the group climbing ladders running up the side of the towering structure. A number of aerial views of central London apparently taken from the top of the Shard were also posted.

Garrett said: "It is impossible to secure a site that big. The security guards that work on the site are only human. No one wants to sit there watching cameras 12 hours a day. They are going to have to take breaks. They are going to have to take walks. "There are going to be times when they are not watching. There are going to be times when they are changing shifts." But the 31-year-old from Los Angeles, who completed a PhD on urban exploration in February, said he and his group had "done with" the Shard for now.

"We do not break anything, we do not alter anything, 90% of the time no one even knows we have been in and out of the place," he said. Garrett, who lives in Clapham, south-west London, said his group had first scaled the building to watch New Year's Eve fireworks over London in December 2010.

Describing the night the pictures were taken, Garrett wrote on his blog that a single security officer appeared to be on duty guarding the £435m building. He said he and two friends had climbed on to the site at 2am from a walkway near London Bridge railway station before scaling the building's staircases to the 76th floor.

"As of December 2011 the Shard claimed the title of 'tallest building in the European Union', stretching 310 metres into the clouds from London Bridge," Garrett wrote.

"It has also been said that is it the most secure site in the city outside of the 2012 Olympic Park. I have never measured the building so I can't testify to the validity of the first claim but I'm happy to respond to the second, as usual."

Describing how they did it, Garrett wrote: "We waited for the guard to finish his current round and go into his hut.

"It took a few minutes of lingering before the walkway was clear of people – we grabbed on to the scaffolding pipes and swung off the bridge.

"Hanging on the freezing pipes, we pulled ourselves on top of the walkway and laid down out of view, waiting for a reaction in case anyone had seen or heard us. It didn't seem so.

"Staying low, we then descended the other side of the scaffolding, right behind the security hut where we could see the guard watching TV, not the cameras. "Quickly, we scampered across the yard and found the central staircase, again pausing to see if there was any reaction from the yard, phones ringing or doors opening. It was silent."

Describing the moment they reached the top of the skyscraper, Garrett wrote: "We were so high I couldn't see anything moving at street level. No buses, no cars, just rows of lights and train lines that looked like converging river systems, a giant urban circuit board."

A spokesman for Sellar Property Group, the developer, and Mace, the main contractor, declined to comment on Garrett's claim that it was still possible to scale the building or that the group had climbed up the building half a dozen times.

"We believe this is an incident which took place around December 2010 when we were undertaking both ground and structure works," he said.

"The breach was discovered very soon afterwards and security immediately tightened.

"Today security on the site is tight with 14 night-time security guards on duty continuously who cover all areas, as well as 25 CCTV cameras in operation together with a ground-floor-level laser alarm system."

Scotland Yard said it had not received any complaints about the alleged break-in.

The Shard has become the tallest building in the European Union and the 45th tallest in the world.

It surpassed the previous record-holder for the capital, One Canada Square at Canary Wharf, which stands at 235 metres (773ft) and was completed in 1991.